Unless there's some super judicial twisting and stretching, confiscating arms that violate laws made after the fact violates the ex post facto/bill of attainder clause in Article I, section 9 of the constitution. Basically, they cannot criminalize behavior, then go back and punish all those individuals who violated it prior to the passage of the law. To do so would violate due process, since there isn't any due notice.

However--it seems that under the Lautenberg laws, firearms may be removed out of the hands of individuals formerly convicted of domestic violence, even if they had the weapons prior to the passage of the Lautenberg laws.